The multi-tasking gut epithelium of insects

Abstract

The insect gut epithelium plays a vital role in multiple processes, including nutrition, immunity and osmoregulation. Recent research is revealing the molecular and biochemical basis of these functions. For example, the pattern of nutrient acquisition by the gut epithelium is integrated into the overall regulation of nutrient allocation, as illustrated by evidence for systemic controls over expression of key genes coding digestive enzymes and transporters in carbohydrate acquisition; and the abundance and diversity of microorganisms in the gut lumen is regulated by multiple molecular properties of the gut epithelial cells, including the synthesis of enzymes that produce reactive oxygen species and anti-microbial peptides. These traits are underpinned by the function of the gut epithelium as a selective barrier which mediates the controlled movement of water, ions, metabolites and macromolecules between the gut lumen and insect tissues. Breakdown of the gut epithelial barrier has been implicated in muscle paralysis of insects at low temperatures (chill coma) and in aging. The key challenge for future research is to understand how the multiple functions of the insect gut epithelium are integrated by signaling interactions among epithelial cells, the gut microbiota and other insect organs.

Keywords: Epithelium; Gut; Immunity; Microbiota; Nutrient assimilation; Stem cell.

Fig. 1

Integration of the digestive and assimilatory function of the gut epithelium into the system-wide regulation of nutrient acquisition and allocation by an insect. White arrows: the multi-way integration of gut epithelium function with feeding and nutrient allocation. Shaded arrows: nutrient allocation to competing fates (here shown as four broad categories) is determined by interaction between nutrient availability and the insect neuro-endocrinal controls.

Fig. 2

Dynamics of microorganisms associated with the insect gut. Most ingested microbial cells pass through the gut lumen with bulk flow of food or are retained or killed in the gut. Transfer to the hemolymph (dashed line) is generallyy prevented by physical (peritrophic envelope) and chemical (immune-related) barrier functions. IMD pathway, immune deficiency pathway; AMPs, antimicrobial peptides; ROS, reactive oxygen species.

Fig. 3

Transport across the gut lumen. Transcellular transport can be mediated by membrane-bound transporters and by transcytosis via endocytic vesicles, and paracellular transport is controlled by septate junctions. Paracellular transport occurs between epithelial cells. Transport may occur from apical to basal surface (shown) or in the reverse direction. EC, enterocyte; EE, endocrine cell; N, nucleus.